linux-kernel - Re: [PATCH] device property: Add scoped fwnode child node iterators

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Message-ID: <AA99AF86-CD2A-4BBE-92BC-3D9005E7BA3B@gmail.com>
Date: Fri, 29 Aug 2025 07:34:17 -0400
From: Jean-François Lessard <jefflessard3@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC: "Rafael J. Wysocki" <rafael@...nel.org>, Danilo Krummrich <dakr@...nel.org>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Daniel Scally <djrscally@...il.com>,
 Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
 Sakari Ailus <sakari.ailus@...ux.intel.com>, linux-acpi@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] device property: Add scoped fwnode child node iterators

Le 29 août 2025 00 h 23 min 22 s HAE, Greg Kroah-Hartman <gregkh@...uxfoundation.org> a écrit :
>On Thu, Aug 28, 2025 at 10:17:59PM -0400, Jean-François Lessard wrote:
>> Add scoped versions of fwnode child node iterators that automatically
>> handle reference counting cleanup using the __free() attribute:
>> 
>> - fwnode_for_each_child_node_scoped()
>> - fwnode_for_each_named_child_node_scoped()
>> - fwnode_for_each_available_child_node_scoped()
>> 
>> These macros follow the same pattern as existing scoped iterators in the
>> kernel, ensuring fwnode references are automatically released when the
>> iterator variable goes out of scope. This prevents resource leaks and
>> eliminates the need for manual cleanup in error paths.
>> 
>> The implementation mirrors the non-scoped variants but uses
>> __free(fwnode_handle) for automatic resource management, providing a safer
>> and more convenient interface for drivers iterating over firmware node
>> children.
>> 
>> Signed-off-by: Jean-François Lessard <jefflessard3@...il.com>
>> ---
>> 
>> Notes:
>>     checkpatch reports false positives that are intentionally ignored:
>>     COMPLEX_MACRO, MACRO_ARG_REUSE, MACRO_ARG_PRECEDENCE
>>     This is a standard iterator pattern following kernel conventions.
>> 
>>  include/linux/property.h | 14 ++++++++++++++
>>  1 file changed, 14 insertions(+)
>> 
>> diff --git a/include/linux/property.h b/include/linux/property.h
>> index 82f0cb3ab..279c244db 100644
>> --- a/include/linux/property.h
>> +++ b/include/linux/property.h
>> @@ -176,6 +176,20 @@ struct fwnode_handle *fwnode_get_next_available_child_node(
>>  	for (child = fwnode_get_next_available_child_node(fwnode, NULL); child;\
>>  	     child = fwnode_get_next_available_child_node(fwnode, child))
>>  
>> +#define fwnode_for_each_child_node_scoped(fwnode, child)		\
>> +	for (struct fwnode_handle *child __free(fwnode_handle) =	\
>> +		fwnode_get_next_child_node(fwnode, NULL);		\
>> +	     child; child = fwnode_get_next_child_node(fwnode, child))
>> +
>> +#define fwnode_for_each_named_child_node_scoped(fwnode, child, name)	\
>> +	fwnode_for_each_child_node_scoped(fwnode, child)		\
>> +		for_each_if(fwnode_name_eq(child, name))
>> +
>> +#define fwnode_for_each_available_child_node_scoped(fwnode, child)	\
>> +	for (struct fwnode_handle *child __free(fwnode_handle) =	\
>> +		fwnode_get_next_available_child_node(fwnode, NULL);	\
>> +	     child; child = fwnode_get_next_available_child_node(fwnode, child))
>> +
>>  struct fwnode_handle *device_get_next_child_node(const struct device *dev,
>>  						 struct fwnode_handle *child);
>>  
>
>We need a real user of this before we can add them, so please do that as
>part of a patch series.
>

I understand the "no dead code" policy, but I found existing manual
implementations of this exact pattern in the current kernel.

For example, drivers/i2c/i2c-core-slave.c already does:

		struct fwnode_handle *child __free(fwnode_handle) = NULL;
...
		fwnode_for_each_child_node(fwnode, child) {
...
		}

This suggests developers are already wanting this functionality but
implementing it manually.

If included with my driver series, I would only add the
fwnode_for_each_available_child_node_scoped() variant that I actually use,
avoiding any dead code.

Best Regards

Jean-François Lessard